The thickness of the gate oxides of metal-oxide-semiconductor (MOS) devices is critical to the performance of the MOS devices. There is a constant need for thinner gate oxides to allow for higher-speed devices. Current technology requires gate oxide thicknesses to be about 50 angstroms or less. However, for ultra-thin silicon dioxide gates, leakage currents will increase tremendously as the thickness is reduced. This will cause a high standby current in the standby mode (IOFF) and high standby power consumption, thereby making products with these devices commercially unacceptable. Therefore, reducing standby power consumption and improving performance are often conflicting requirements.
In high end products, there is a need to form devices having multiple gate oxide thicknesses on a same chip. For example, circuits for input/output (I/O) connections, circuits with high performance, and circuits with low power consumption must be fabricated on a same chip, each having a gate oxide thickness, hence the respective chip is referred to as a triple gate oxide chip. The use of triple gate oxides satisfies requirements of both low standby power consumption and high performance. For example, low power-consumption circuits may have thick gate oxides, and thus having low standby currents and low leakage currents. High performance circuits, on the other hand, have thinner gate oxides, and thus have higher speeds.
Triple gate oxide chips, however, require triple power supplies with triple power supply voltages in order to achieve desired effects. Particularly, problems arise if the high performance circuits and the low standby power-consumption circuits both include static random access memory (SRAM) cells and share a common power supply voltage. If the desired power supply voltage of high performance circuits, which supply voltage is typically low, is used for both circuits, the SRAM cells in the low standby power-consumption circuits may not be able to function correctly due to inadequate read and/or write margins. If, however, the power supply voltage is increased to allow the low standby power consumption circuits to work correctly, the power consumption on the high performance circuits is undesirably increased. Accordingly, a solution is needed.